With her new book "Lean In: Women, Work, and the Will to Lead," Facebook Chief Operating Officer Sheryl Sandberg clearly hoped to spark discussion about female ambition and opportunity in the workplace. She might not have expected her observations and prescriptions to bring her the kind of criticism she's received for holding women to standards, both personal and professional, that many see as unattainable.

But whatever one's reaction to "Lean In," it has brought into focus, once again, the disparity between the number of men and women who hold leadership roles in major businesses. It's a reality faced by many female heads of Silicon Valley startups, both new and veteran.

They have seen the issues that Sandberg has raised play out in their own experiences, and though they can't pinpoint one surefire solution, many agree about the need to form support groups, seek out role models and get more girls interested in math and science earlier.

Ann Scott Plante, 29, co-founder of Wello, a video-based workout website started in July, said her goal as a female executive is to make high-achieving women like Sheryl Sandberg no longer the exceptions.

"It's worth calling out situations where it doesn't seem like a level playing field," Scott Plante said. "We should aspire to do everything we can to play with the boys and make a more friendly environment for women."

Workplace parity

A look at recent data on the topic of workplace parity in upper echelons underscores Sandberg's concerns. A 2012 report from executive search firm Spencer Stuart points out that top leadership positions are still overwhelming held by men.

"The slow progress is discouraging," the report says. "Women now account for just over 17 percent of independent directors, up from 16 percent in 2007 and 12 percent in 2002. Nine percent of S&P 500 companies do not have any female directors." A mere 21 of the Fortune 500 chief executives are women.

Matthew Bidwell, an assistant professor of management at the University of Pennsylvania, wrote an article for a Wharton business school publication on gender segregation in managerial roles. He focused on professional women entering the workforce, and found that fewer who finished MBA programs chose investor-type jobs, because those positions require more stereotypically male attributes and attitudes.

"At least at lower levels, I found the positive thing is that women aren't being discriminated against," Bidwell said. "But at the same time, managing stereotypes is harder than managing discrimination."

Catherine Wolfram, an associate professor at UC Berkeley's Haas Business School, studied the "leaky pipe" theory, which says that women aren't staying in positions long enough to rise through the ranks. Her study found that 15 years after graduating from Harvard College, 28 percent of the females who received MBAs were stay-at-home moms, while only 6 percent of those earning medical degrees left their jobs, suggesting the business world is less female-friendly.

Stereotypes persist

For women in leadership in Silicon Valley, negotiating the path to their roles has been a constant learning experience, some of it unpleasant.

Jamie Walker, 30, CEO of online health community Fit Approach, is releasing the fitness class app Sweat Guru this spring. She recalled once inquiring of a panel of venture capitalists how many female startups were in their portfolios. One panelist responded: "We're not looking at whether you have boobs are not."

"It wasn't really the comment, but the reaction that was troubling, because people just thought it was funny," said Walker's co-founder, Alyse Mason Brill, 29. "Even when we go into business meetings, there are subtle things, too - like you can see them looking at you and not taking you as seriously because of your feminine traits. I think we've seen the way to push through this is to not back down."

Lynn Jurich, 33, co-founder of Sunrun, a home solar company, said her first job out of college, in 2002, was at a venture capital firm, Often, people she dealt with were surprised she was in her position.

"One time I was going to interview someone for a position at the company and he said, 'Oh, I already have coffee, I'm all set,' " Jurich said. Another time, "I was on the company's private jet and the pilot would say, 'Your dad must own the company.' "

Myth of balance

Deeply ingrained gender stereotypes clearly play a part in the dearth of top women leaders. Tending to be the primary caregivers for children, women are often pulled away from the workplace, thus limiting access to demanding corporate jobs.

But not all agree it has to be a hindrance.

Monisha Perkash, 38, is CEO of Lumo BodyTech, a Palo Alto company that received $5 million in December for its back posture product Lumoback. She says that the notion of work/life balance being a zero-sum game for women - choosing work or children - is a myth when it comes to leading a company. She said she's been able to balance being a parent and running her own startup because she established convergence between her work and her personal life.

Many women agree that having role models is vital for advancing to top positions. Online personal shopping startup Stitch Fix's CEO Katrina Lake, 30, who just raised $4.75 million, said it gets lonely as a CEO because you don't have peers.

DBL Investors managing partner Cynthia Ringo, 60, said sometimes she feels like the Lone Ranger in the male-dominated investment world. She believes that young girls need more relevant experiences - through math, science and business training - to gain the credibility to move forward into CEO-type positions.

Progress in Europe

The Spencer Stuart report points out that European nations have seen much more progress in diversifying their top business ranks. In fact, companies in the S&P 500 now trail the top companies in Norway (40 percent), Finland (27 percent), Sweden (26 percent), France (22 percent), Denmark and the Netherlands (18 percent), tying with Germany (17 percent) for the percentage of females on their corporate boards. Some of these nations have adopted regulations or set voluntary targets to increase these numbers.

But some women are leery of change through regulation.

"I'm torn when it comes to the government getting involved in this problem," said Laurie Yoler, a veteran Silicon Valley technologist. "Clearly it's not happening on its own, so the question is: Is regulation a requirement" to make it happen?

Changing behavior

So what can be done?

Penn's Bidwell suggests that employers can reduce the work and family conflicts many women face with more flexible work schedules and access to day care. They can also present job descriptions in more gender-neutral terms.

Katia Beauchamp, co-founder of the commerce platform Birchbox, which has 300,000 subscribers, says women leaders can reinforce their own effectiveness. Each time they present to venture capitalists in particular, she said, it's important to focus on how they plan to create demand for something new and change behavior.

Yoler says that better media representation of engineering jobs is one of the keys to changing the number of women who are attracted to engineering and computer science degrees, the kind of educations that lead to technology careers.

Films like "Zero Dark Thirty" and "The Girl with the Dragon Tattoo" feature women in these fields, she points out, but the majority of career women represented in the media are seen in the roles of doctors or lawyers rather than hackers.

Diversity of thought

Yoler believes it's important to have more women in executive-level roles for reasons essential to good business. Women can add a diversity of thought and strategic insight, she said. They may also offer a different customer perspective, and often bring a different kind of management style.

Perkash agrees, saying that more companies have to realize that "to be competitive, you have to be open to half the workforce if you want your company to do better. Otherwise, (they) will be missing out on a big part of the talent."

Until then, she suggests a route she has followed.

"My advice to women is that entrepreneurship is a good route to take to try to have it all," she said. "They won't hit glass ceilings because they can make up the rules."